1. Field of the Invention
This invention relates to a glass article having a water-repellent surface having excellent durability. In particular, a coarse surface silica layer having micro-crystal silica particles is formed on the glass substrate. On top of a surface, fluoroalkylsilane (FAS) is applied, forming such water-repellent layer. More particularly, this invention relates to such water-repellent glass having a superior durability and water repellent property, and to a process for manufacturing the same, in which long-term use does not induce deformation in appearance and deterioration in water-repellent property.
2. Background of the Invention
In the case of various types of glasses exposed to an external environment, such as safety glass windows for automobiles, building glass windows, and mirrors the surface of such glass articles frequently becomes contaminated with rain or some other moisture, resulting in a poor visual field. In order to solve that problem, a water-repellent property conventionally has been added to the surfaces of such glass articles.
In general, to provide a water-repellent property to the surface of a glass article, the surface energy at the glass surface should be lowered. To this end, a substance having low surface energy (hereinafter referred to as "water-repellent agent") should be placed on the glass surface. In line with this, a common type of soda lime glass plate shows a contact angle of about 20.degree. to water, but a glass sample with excellent water-repellent treatment shows a contact angle of more than 100.degree..
As prior art, "RAIN-X" (Unelko Corp. USA, U.S. Pat. No. 3,579,540), the currently commercialized process for coating a silicon-based water repellent agent onto the surface of a glass substrate is the most well-known process via-a-vis adding a water-repellent property to the surface of a glass substrate via coating with a water-repellent agent. In general, hydrocarbon-based silicon-based, chlorine, a fluorine compounds are being used as water-repellent agents, for such uses. Among them, hydrocarbon- and silicon-based compounds have a critical surface tension of about 30 dyn/cm, and fluorated alkyl compounds (hereinafter referred to as "Rf compounds") containing CF3 and CF2 groups within the respective molecules have a critical surface tension of less than 20 dyn/cm, thus effectively repelling water, polar solvent and even oil.
Hitherto, the fluoroalkylsilane-based water-repellent agents have been reported as having the best water-repellent property among the conventional water-repellent agents since they have an ample amount of CF3 or CF2, a substance providing the lowest surface energy. Further, the corresponding linear structure is quite advantageous in increasing the density of water-repellent group.
A water-repellent agent for, providing the water-repellent property to glass, has a molecular site with tendency to induce a strong siloxane bond (Si--O--Si) by reacting with a silanol (OH) group at the glass surface and an Rf group with a fluorocarbon group providing a hydrophobic property at the other side contacting air. With such structural characteristics, various methods of providing a water-repellent property on a glass surface using Rf compounds have been developed as the most practical approach. However, in the case where these fluorine compounds are applied to soda lime glass, the dissolution of alkali within the glass is responsible for degrading the water-repellent property, with the passage of time.
Therefore, it is necessary that a fine and firm silica film be coated on the surface of the glass substrate that is to be made water repellent prior to applying the water-repellent film, in conjunction with inhibiting the dissolution of alkali substances within the glass. In particular, in the case of the automobile glass which is vulnerable to the external environment due to ceaseless surface friction and contamination, its water-repellent capability from the direct treatment by water-repellent agent is insufficient. Thus, additional factors for durability should also be considered. Durability as applied here means resistance from abrasion, scratching, rain, wind, chemical substances, sun rays, ultraviolet and temperature, etc. Namely, such durability allows the continuation of the water-repellent property for a long period of time, for the enjoyment of the functional benefits of the water-repellent glass.
In the past, many inventors have extended their intensive studies to improve the durability of water-repellent glass in various aspects. Among the conventional methods, for the purpose of providing durability to the water-repellent film, the technology of forming a sub-layer film, comprising highly adhesive and durable materials, at the surface of the desired substrate prior to forming a water-repellent film has been in common usage in the field of coating technology, using the sol-gel method. Namely, it is a method of double film treatment in which a silica layer using tetraethoxysilane (TEOS) is first formed at the surface of the glass substrate, and then, a water-repellent film comprising an Rf compound is formed thereon. In addition to TEOS, other metal oxides such as Al.sub.2 0.sub.3, TiO.sub.2, ZrO.sub.2, ZnO.sub.2 or ITO may be selected use as the silane compound.
However, if the water-repellent agent is directly applied to the plain glass and the silica surface, the adhesion area to the slippery surface is relatively small. Thus, removability of the water-repellent agent from the surface cannot sustain the durability and water-repellent property.
To solve this problem, the Japanese Unexamined Patent Applications Hei No. 4-325446 and Hei No. 5-24885 disclose methods of enhancing the durability in such a manner to form fine pores within a silica layer, by mixing polyethylene glycol and triethylene glycol monoethylene ether when forming the silica layer. Further, the Japanese Unexamined Patent Applications Hei No. 4-124047 and Hei No. 6-116430 disclose methods of forming at least one transparent metal oxide layer at the glass surface and a modifying the surface thereafter via wet etching or plasma discharge etching, and then finally forming the water-repellent layer on the of the surface as so modified.
However, the fine pores within the silica layer formed by the methods of the Japanese Unexamined Patent Applications Hei No. 4-325446 and Hei No. 5-24885 are not stable during thermal treatment process at temperatures of more than 400.degree. C., thereby unduly restricting the thermal treatment process. Furthermore, the methods disclosed by the Japanese Unexamined Patent Applications Hei No. 4-124047 and Hei No. 6-116430 have the disadvantage of requiring an additional etching process after the formation of the silica layer.
Meanwhile, the Japanese Unexamined Patent Application Hei No. 5-147976 discloses a metal oxide manufacturing method which provides an ample amount of fine coarse surface via the sol-gel method using a sol-gel which is prepared from and obtained by mixing two sols having different molecular weights, and using metalalkoxide or methylacetylacetonate.
Nevertheless, such methods have disadvantages and require improvement in that a) the durability effect which they provide is not satisfactory, and b) the processes are complicated so that there a small economic benefits.